The techniques of redox potentiometry, electron paramagnetic resonance (EPR) spectroscopy and Mo K-edge X-ray absorption spectroscopy have been applied to wild-type human and a series of mutant human sulfite oxidases, including two clinical mutants. The combination of redox potentiometry and EPR spectroscopy has allowed us to define precise conditions for redox poising samples to be examined by X-ray Absorption spectroscopy, allowing us to examine the active site structures of the Mo(VI), Mo(V) and the Mo(IV) oxidation states. Extensive data have been collected on a large number of samples and detailed analysis is now underway. We expect to be able to identify the functional role within the catalytic cycle for several amino acids near to the molybdenum active site.